MECHANISMS OF TRANSMEMBRANE ELECTRON-TRANSFER - DIFFUSION OF UNCHARGED REDOX FORMS OF VIOLOGEN, 4,4'-BIPYRIDINE, AND NICOTINAMIDE WITH LONGALKYL CHAINS

Transmembrane electron transfer in lecithin (phosphatidylcholine) vesi cles was studied by pulse radiolysis. Upon reduction, cetylmethylviolo gen (N-hexadecyl-N'-methyl-4,4'-bipyridinium CMV), cetylbipyridine (4- (N-hexadecylpyridinium-4-yl)pyridine, CB), and cetylnicotinamide (N-he xadecyl-3-(aminocarbonyl)pyridinium, CNA) transferred electrons from t he bulk water phase to Fe(CN)63- in the internal water phase of the ve sicles. The transmembrane electron transfer was found in all cases to proceed through diffusion of uncharged forms of the redox mediators (C MV0, CB0, and CNA0, respectively) but the kinetic behavior varied cons iderably. The mechanisms for CB and CNA were simple, the reaction foll owing first-order kinetics, and the transmembrane diffusion was rate l imiting (k = (1.5 +/- 0.3) x 10(3) S-1 for CB and k = 3.2 +/- 0.5 s-1 for CNA). The mechanism for CMV was more complicated, and the reaction followed second-order kinetics. The rate-determining step was propose d to be the disproportionation of two viologen radicals formed by the radiation pulse (2CMV+ double-line arrow pointing left and right CMV0 + CMV2+), followed by rapid transmembrane diffusion of CMVO and its su bsequent reoxidation by Fe(CN)63-. In pulse radiolysis, and in phospho rescence quenching experiments with Pt2(P2O5)4H84-, CB0 and CB+ were u sed as models in order to obtain the rates of transmembrane diffusion of CMV0 and CMV+, respectively. Our results exclude the possibility of electron tunneling between viologens on opposite sides of the membran e, and they provide strong arguments against transmembrane diffusion o f viologen radical (CMV+).